The present invention relates to the manufacture of chlorine and sodium carbonate. More particularly, it relates to a method for the manufacture of chlorine and sodium carbonate by a combination of the electrolysis of an aqueous solution of sodium chloride in diaphragm cells with the carbonation of the caustic brine leaving the diaphragm cells and with the ammonia-soda method of making sodium carbonate.
It is known that in diaphragm cells for the electrolysis of aqueous solutions of sodium chloride, the proportion of the sodium chloride decomposed is less than 100% so that the caustic liquor obtained has a content of unconverted sodium chloride that is always more or less high, such a liquor being sometimes called "caustic brine". In current practice, because of the cost of electrical energy and in the interests of current efficiency, the conversion of sodium chloride during electrolysis in diaphragm cells is limited to about 50% of the sodium chloride starting material and the relatively dilute solutions obtained must be concentrated in multiple effect evaporators coupled with salt separators. The products obtained are chlorine, hydrogen, caustic liquors and salt, the last being reusable in concentrating the brine sent to electrolysis (cf. R. N. Shreve, The Chemical Process Industries, 2nd Edition, 1956, page 300).
The ammonia-soda method of making sodium carbonate (cf. R. N. Shreve, page 289 to 295 of the above reference) may be summarized as follows.
1. Formation of brine purified with respect to calcium and magnesium ions, usually by the action of sodium carbonate and lime or sodium carbonate and caustic soda on raw brine.
2. Use of purified brine for washing the exit gas from the bicarbonation columns.
3. Ammoniation, in apparatus known as an "absorber", of the brine leaving the gas washer by absorption of the ammonia and the carbon dioxide liberated in a later phase called "distillation".
4. Treatment of the ammoniacal brine thus produced in bicarbonation columns working in series with a bicarbonation column involved in the preceding operations, the latter column being known as a "washing column". The bicarbonation columns, furnished with trays or passettes and refrigerated at their lower part, are fed with carbon dioxide of 90 - 95% strength obtained by the calcination of sodium bicarbonate and with gas from lime kilns while the washing column is fed with gas of low CO.sub.2 content. Crude sodium bicarbonate precipitates on formation in the bicarbonation columns.
5. Filtration of suspension leaving the bicarbonation columns in order to separate the crude sodium bicarbonate, which is washed with water on the filter in order to free it from sodium chloride and ammonium chloride. The mother liquor from the filtration consists of an aqueous solution containing principally sodium chloride and ammonium chloride.
6. Treatment of this mother liquor, in a stage known as distillation, to recover carbon dioxide and ammonia which is returned to the absorber for ammoniation of the brine. The step known as distillation consists in the main in heating the mother liquor from the filtration in order to separate the carbon dioxide, adding lime and treating the liquid obtained in a distillation column at the base of which steam is injected. The lime reacts with the ammonium chloride to form ammonia and calcium chloride, and the steam entrains the liberated ammonia.
7. Optionally, the liquor having the distillation -- which consists essentially of an aqueous solution of calcium chloride and sodium chloride -- may be submitted to evaporation in stages to recover the two chlorides separately.
8. The crude sodium bicarbonate obtained from the filter is generally calcined in a rotating drum drier ("secheur") in order to form anhydrous sodium carbonate (light ash) and a gas rich in carbon dioxide, which is cooled, purified, compressed and passed to the bottom of the bicarbonation columns.
9. If a sodium carbonate of specifically greater density (heavy ash) is required for certain uses, the light ash is mixed with sufficient water to form sodium carbonate monohydrate and this is then dried in a rotating kiln. According to a previously proposed process (French Patent 1,350,230), sodium carbonate monohydrate suitable for conversion to heavy ash may also be made by direct reaction of the crude bicarbonate removed from the filter of the soda plant with caustic liquor and anhydrous sodium bicarbonate, in this way the calcination of crude bicarbonate to light ash and rehydration of the light ash are dispensed with.
In an ammonia-soda plant of the conventional type, such as described above, the proportion of the sodium chloride converted is generally of the order of 72 - 76%.
In French Patent 1,364,282 there is described a process for the manufacture of sodium carbonate and chlorine wherein caustic brine from diaphragm cells is pre-carbonated without precipitation by means of a gas of low carbon dioxide content so as to convert the caustic soda to sodium carbonate and is then passed to an ammonia-soda plant together with ammoniacal brine prepared in the conventional manner. This procedure allows the addition of supplementary amounts of ammonia with the object of bicarbonation, which results in a higher percentage conversion of sodium chloride to soda and a reduced consumption of steam and lime. As indicated in other patents (French Patents 1,514,501 and 2,006,968), the formation of precipitates in the absorber, the washing column and the upper part of the bicarbonation columns is avoided when using caustic brines from electrolytic cells by recycling part of the liquid from the filtration stage to the absorber, optionally, after reducing its carbon dioxide content.
As the needs of the market have developed, it has been found that there is a tendency to produce an excess of caustic soda. There is therefore a need for processes that allow production of chlorine with co-production of sodium carbonate rather than co-production of caustic soda. Moreover, it is of importance to preserve as long as possible the existing ammonia-soda installations by altering them as necessary because of at least partial modification of the raw materials employed. Finally, according to another aspect of the problem, it is indispensable to increase the proportion of sodium chloride converted or to allow the recovery of unconverted chloride so as to reduce to a minimum the pollution of rivers.